1. Field of the Invention
The present invention relates to a method for the removal of metallic impurities from aqueous acidic nickel or cobalt solutions. More particularly, the invention relates to a method of removing zinc, copper and/or iron impurities from acid nickel electroplating solutions.
2. Prior Art
Plating processes differ in sensitivity to the presence of metallic impurities within the electroplating bath. Thus, for example, while stannate tin is extremely tolerant of most impurities, bright nickel and most cobalt plating solutions are sensitive to the presence of impurities. Various means have been devised for purifying aqueous metal solutions by chemical treatment or with the use of mechanical means. One conventional technique, for example, for purifying electroplating solutions is the process known as "dummying". This consists of plating for a period of time and usually at low current densities upon cathodes which are not intended for use, such as pieces of scrap sheet metal. This low current density electrolysis plates out metallic impurities and/or decomposes some organic contaminants.
Another method for the removal of contaminants from metal solutions consists of passing the solution, intermittently or continuously, through a filter or a series of such devices. This technique, while satisfactory for insoluble impurities, is not adequate for the removal of dissolved impurities. Problems of this type have been overcome with the use of additive materials which react with or complex with the soluble impurities. An operation of this general type is disclosed by Michael in U.S. Pat. No. 3,257,294. While the technique is effective for the removal of soluble metallic impurities, a difficult problem exists for each individual application in locating a composition that can be added to the metallic solution without further increasing impurity problems and which will preferentially react or complex with the soluble metallic impurities to form relatively insoluble compositions that can be subsequently removed using filtration techniques.
These two aforementioned methods have been already overcome and improved upon by the method described in U.S. Pat. No. 3,518,171 issued June 30, 1970 and assigned to the assignee of the present invention. The substances to be added to the bath and disclosed in this last mentioned U.S. patent are, however, selected di-alkyl-dithiocarbamate salts and their utilization is described as applicable to nickel electroplating baths.
It is, therefore, an object of this invention to provide a new class of compounds for the removal of soluble zinc, copper and/or iron impurities from aqueous nickel or cobalt solutions, utilizing the technique disclosed and claimed in said U.S. Pat. No. 3,518,171.